1. Field of the Invention
This invention relates to a semiconductor memory device, and more particularly, to an electrically erasable and programmable read-only memory (hereinafter referred to as "EEPROM") such as a flash memory.
2. Description of the Related Art
In EEPROMs, storage of information is generally effected by accumulating charges in a conductor called a "floating gate" which is disposed in a transistor (a so-called "cell transistor") of a memory cell portion, and which is electrically insulated from other portions.
Therefore, when leak occurs due to breakage or defect of an insulating film surrounding the floating gate, the charges accumulated in the floating gate disappear and the information is destroyed.
When EEPROMs are shipped from a production plant, writing and reading of information are effected, and EEPROMs which include any defective cell transistor or transistors are rejected. Furthermore, after information is stored in the EEPROMs, they are left lying in a high temperature environment for a predetermined period, presence of the loss of information is checked, and only EEPROMs free from the loss of information are shipped.
However, since rewriting of the EEPROM is made on the user side during use, too, leak resulting from breakdown of the cell transistor and from deterioration of the insulating film surrounding the floating gate occurs, and there is the possibility that the charges accumulated disappear during use and the information is destroyed.
When writing and erasing are repeatedly effected for the cell transistor, a so-called "disturbing", in which the information of other cell transistors disappears due to the influences of a voltage applied to a given cell transistor, occurs, particularly in a flash memory which has become predominant in recent years.
Breakdown of the cell transistor mostly occurs at the time of rewriting, and can be detected by reading out the information immediately after the information is re-written. However, since the loss of charges of the floating gate due to leak and disturbing gradually develops, it cannot be detected by reading out the stored information of the cell transistor immediately after the rewrite operation. The loss of the information is mostly found out when the stored information of the cell transistor is later read out, and thus repair of the information is difficult.
Conventionally, repair of the information which is destroyed during use has been carried out by incorporating an ECC (Error Check and Correct) circuit into EEPROMs.
When the ECC circuit is built in, the major portion of random defects of the cell transistors can be relieved, but there are posed the problems that excessive cell transistors become necessary, peripheral circuits inside the device become more complicated, and the increase of the device area and its fabrication cost are invited.